Influence of the dielectric and of the active layer doping on the FET mobility in PPV-based devices

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on Poly-phenylenevinylene derivatives. We observe a direct influence of the dielectric surface on the field-effect mobility as well as on the charge injection at the source electrode, despite the fact that we used a top contact transistor structure. We find that the presence of traps at the dielectric surface, decreases the hole mobility and increases the threshold voltages. By treating the silicon dioxide dielectric surface with gas phase molecules such as octadecyltrichlorosilane (OTS) and hexamethyldisilazane (HMDS) the hole mobility improves and the threshold voltage slightly increases. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related to the density of the oxydryl groups (-OH), which are the most efficient traps for the charges flowing in the device. We use different polymer species such as polyvinylalchol (PVA), polymethylmetacrilate (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB). The elimination of the -OH groups and of other traps, produces the same effect observed with HMDS coupled to a more pronounced enhancement of the threshold voltage, with the exception of PMMA. The electrical characteristics obtained with HMDS and PMMA polymer dielectrics are the highest reported to date for PPV-based field-effect transistors. We confirm that the purification of the active material is crucial to enhance the device performances and to achieve a better device to device reproducibility. We also investigated the effect of the dispersion of a phosphorescent dye into the active polymeric material. The electrical characteristics of OFETs with HMDS or PMMA dielectric with and without dye doping are compared.